1. Field of the Invention
This invention relates generally to a system and method for purging excess water from a fuel cell stack and, more particularly, to a system and method for purging excess water from a fuel cell stack at system shut-down that includes providing a forward or reverse cathode side air flow purge and/or a reverse hydrogen flow through the anode flow channels to push water from outlet ends of the anode flow channels.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode side catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode side catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode electrodes (catalyst layers) typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). Each MEA is usually sandwiched between two sheets of porous material, a gas diffusion layer (GDL), that protects the mechanical integrity of the membrane and helps in uniform reactant and humidity distribution. The part of the MEA that separates the anode and cathode flows is called the active area, and only in this area the water vapors can be freely exchanged between the anode and cathode. MEAs are relatively expensive to manufacture and require certain humidification conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a reaction by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
A fuel cell stack includes a series of bipolar plates (separators) positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include anode side and cathode side flow distributors (flow fields) for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The membranes within a fuel cell need to have certain water content, or humidification, so that the ionic resistance across the membrane is low enough to effectively conduct protons. This humidification may come from the stack water by-product or external humidification. The flow of hydrogen through the anode gas flow channels has some drying effect on the membrane, most noticeably at an inlet of the hydrogen flow. However, the accumulation of water droplets within the anode gas flow channels could prevent hydrogen from flowing therethrough, and cause the cell to fail because of low reactant gas flow, thus affecting the stack stability. The accumulation of water in the reactant gas flow channels as well as within the GDL is particularly troublesome at low stack output loads.
During fuel cell system shut-down, it is desirable to provide the cell membranes with a certain amount of water content so they are not too wet or too dry. A membrane that is too wet may cause problems during low temperature environments where freezing of the water in the fuel cell stack could produce ice that blocks flow channels and affects the restart of the system. Membranes that are too dry may have too low of an electrical conductivity at the next system restart that affects restart performance and may reduce stack durability.
It is known in the art to purge excess water from the flow channels in a fuel cell stack at system shut-down by forcing compressor air through the cathode flow channels and hydrogen gas through the anode flow channels. The duration of the purge and the speed of the gas flow are selected so that the excess water is removed from the flow channels and GDL, but the membranes do not become too dry. One problem with such a purge approach is that there may be a spread of resistance of the cells in the fuel cell stack, meaning some of the cells may be too dry and others may be too wet. Also, in each individual cell an inlet to outlet (mainly defined by cathode flow) water content (resistance) gradient can be observed. Further, purging the anode flow channels with hydrogen gas wastes hydrogen fuel.
It has been proposed in the art to eliminate the anode side purge using the hydrogen gas and use only the cathode side purge using compressor air. During the cathode side purge, water is removed from cathode channels and cathode side GDL. Water is also drawn from the anode side GDL and flow channels through the cell membranes, which typically is effective in removing enough water from the anode side of the active area. However, the parts of the anode and cathode flow fields directly adjacent to the cell inlet and/or outlet normally are outside of the active area, so no water vapor exchange through the MEA is possible. Therefore, when the cell (stack) purge is performed only on the cathode side, all water that has accumulated at the very exit of the anode channels cannot be removed. Thus, one can expect problems restarting the stack, particularly during subfreezing temperature conditions when ice at the anode outlet may block the hydrogen flow through the cell.